There is a heat pump apparatus that heats water using heat absorbed from ambient air. There is a heat pump water heater that heats water in a tank using the water heated by the heat pump apparatus, and there is a heat pump heating apparatus that performs a heating operation using the water heated by the heat pump apparatus.
In the heat pump water heater, when the temperature of the water in the tank reaches the target temperature, the operating frequency of a compressor included in the heat pump apparatus is lowered to reduce power consumption. Likewise, in the heat pump heating apparatus, when the temperature of a room to be heated reaches the target temperature, the operating frequency of the compressor included in the heat pump apparatus is lowered to reduce power consumption.
When the operating frequency of the compressor is lowered, this reduces the flow volume of a refrigerant circulating in a refrigerant circuit included in the heat pump apparatus. With the reduced flow volume of the refrigerant, the pressure of the refrigerant cannot be lowered much even if the opening degree of an expansion valve included in the heat pump apparatus is set to a minimum degree. When the pressure of the refrigerant cannot be lowered, this means that the evaporation temperature of the refrigerant cannot be lowered. Therefore, when the ambient air temperature is low, the evaporation temperature of the refrigerant may not be able to be lowered sufficiently compared to the ambient air temperature.
When the evaporation temperature of the refrigerant cannot be lowered sufficiently compared to the ambient air temperature, the refrigerant cannot sufficiently absorb heat from the ambient air. As a result, the refrigerant cannot be completely evaporated, and the liquid refrigerant is drawn into the compressor. When the liquid refrigerant is drawn into the compressor, a lubricating oil present in the compressor is brought out of the compressor, so that lubricity is reduced and the compressor fails to operate properly.
Conventionally, when the evaporation temperature of the refrigerant cannot be lowered sufficiently compared to the ambient air temperature, the operating frequency of the compressor is increased, thereby increasing the flow volume of the refrigerant circulating in the refrigerant circuit, so as to prevent the liquid refrigerant from being drawn into the compressor (see Patent Literature 1).